[confmap/provider/secretsmanagerprovider] Add support for default values

[theSuess]

Description

This PR adds support for default values when using the secretsmanager provider.
It uses the same syntax as the envprovider and falls back to the default value when:

• the selector is empty
• the json key is not found in the secret map

In both cases, a warning is logged to call out unintentional fallback.

By nesting substitutions, this allows users to specify either a secret manager reference or plain value through environment variables like this:

password: "${secretsmanager:${env:PASSWORD_ARN}:-${env:PASSWORD}}"

When PASSWORD_ARN is set, the collector retrieves the value from AWS Secrets Manager, falling back to the value specified in PASSWORD if the secret is not found or the json key in PASSWORD_ARN is undefined.

Testing

• Manual testing performed using a custom build of the opentelemetry-lambda extension layer.
• Added unit tests

Documentation

• Provider documentation & go API docs

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[theSuess]

@atoulme @dashpole this is still relevant - would appreciate a review

[dashpole]

cc codeowners @driverpt @atoulme

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[theSuess]

Still relevant, rebased onto latest main now

[songy23]

Tests and linter are failing, please fix

[theSuess]

Rebased the PR and fixed the previous linter issues