[exporter/datadog] Move Config structs to main package and reduce API

[mx-psi]

What will change?

The Config structs will be moved to the main package, and will only have Unmarshal and Validate as public methods. In particular:

• The OnceMetadata method will be removed from Config,
• The GetHostTags method will be removed from TagsConfig.
• The GetCensoredKey method will be removed from APIConfig.
• The Sanitize method will be removed from Config, and its functionality will be put into Validate and Sanitize.

Additionally, enum-based configuration settings will use the enum value instead of a string.

Who is affected?

Only users of the Go API will be affected. End-users of the binaries won't be affected.

What should I do if I am affected?

These functions and structs are used internally by the Datadog exporter and the OpenTelemetry Collector, but we don't expect external users to be using them (at least for now!). If you are using them, please reach out.

When will it change?

Versions in italics are tentative

Change description	Version of deprecation	Version of removal
Remove OnceMetadata method	v0.48.0	v0.49.0
Remove GetCensoredKeys method	v0.48.0	v0.49.0
Remove GetHostTags method	v0.49.0	v0.50.0
Remove Sanitize method	v0.54.0	v0.55.0
Move Config structs to main package	v0.55.0	v0.56.0