Deprecate TextMapGetter.keys()

[cfredri4]

Is your feature request related to a problem? Please describe.
TextMapGetter.keys is only used by the deprecated OT trace and Jaeger propagators? But all TextMapGetter implementations needs to implement it, and it's existence means that TextMapGetter.get cannot be inlined as a lambda.

Describe the solution you'd like
Deprecate TextMapGetter.keys and give it a default implementation.

[jack-berg]

Gotta deprecate it at the spec level first: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#keys

[cfredri4]

@jack-berg thanks, opened issue open-telemetry/opentelemetry-specification#5150