Add data source for fetching the Vault address from the provider

[tmatilai]

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Add vault_address data source for fetching the URL of the Vault server configured for the provider.

The data source can be useful if the address is configured for the provider with the VAULT_ADDR environment variable, or if the URL is needed in a child module using the provider. In my use case for configuring redirect URLs of an OIDC IdP.

Release note for CHANGELOG:

Add `vault_address` data source for fetching the URL of the Vault server: ([#1566](https://github.com/hashicorp/terraform-provider-vault/pull/1566))

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccDataSourceAddress'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -run=TestAccDataSourceAddress -timeout 30m ./...
?       github.com/hashicorp/terraform-provider-vault   [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/coverage      [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/generate      [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/codegen   (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/generated [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role        (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation      (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/helper    [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/consts   [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/internal/identity/entity  (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/group   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/pki      [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/internal/provider (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/testutil  (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
2022/08/05 17:01:26 [INFO] Using Vault token with the following policies: admin, default
=== RUN   TestAccDataSourceAddress
--- PASS: TestAccDataSourceAddress (2.47s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     (cached)

[hashicorp-cla]

All committers have signed the CLA.

[tmatilai]

Any chance that this could be considered/reviewed? 🥺 It would avoid passing the URL as a variable to child modules.

I can rebase onto current master when requested.