Add external_access_enabled parameter to Metastore API (Fixes #1200)

[Krishnan9074]

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What changes are proposed in this pull request?

This PR adds support for the external_access_enabled parameter in the Databricks Metastore API to resolve issue #1200.

WHAT:

• Added the ExternalAccessEnabled field to the UpdateMetastore struct in service/catalog/model.go
• Updated the example test in service/catalog/metastores_usage_test.go to demonstrate usage of the new field
• The field is properly tagged with json:"external_access_enabled,omitempty" for API compatibility

WHY: This change is needed because the current implementation of the Databricks metastore Terraform resources does not support the option of external_access_enabled. This parameter is documented in the Databricks API reference and allows or prevents non-Databricks Runtime clients from directly accessing entities under the metastore.

Adding this field will enable Terraform users to control this important security setting through the Databricks Terraform provider, as requested in issue #1200.

How is this tested?

The change has been tested by:

1. Building the code with go build -mod vendor ./... to verify compilation
2. Updating the example test to include the new field in the UpdateMetastore call

The full end-to-end functionality would be tested when integrated with the Databricks Terraform Provider, but this PR only implements the necessary SDK changes to enable that integration.

Fixes #1200