Add Sigv4 Endpoint config parameter

[kanwren]

Exposes an endpoint field in the sigv4 config, so that the call to STS when assuming a role with a web identity can use a custom STS endpoint, e.g. a VPC endpoint.

No clients other than the STS one are made from the session config here, so setting Endpoint should be safe, but it can also be a resolver if this is a concern.

Addresses https://github.com/prometheus/prometheus/issues/11710.

[qswinson]

Any chance on getting this merged? It would be very helpful for AWS VPC deployments that need to keep all network traffic within AWS.

[rapphil]

Thanks for creating this PR.

[alexgreenbank]

Hello from the bug scrub.

There is some feedback on this PR that needs attention. Is there any further work required?

[SuperQ]

This package has been migrated to https://github.com/prometheus/sigv4.