Add `GetAll` to `TextMapPropagator` Extraction

[MrAlias]

Support the stabilized specification:

GetAll

For many language implementations, the GetAll function will be added after the stable release of Getter.
For these languages, requiring implementations of Getter to include GetAll constitutes a breaking change
since instrumentation which previously functioned would fail. Language implementations should be cognizant
of this, and add GetAll in a way that retains backwards compatibility. For example, by providing a default
GetAll implementation based on Get, or by creating an extended Getter type.

If explicitly implemented, the GetAll function MUST return all values of the given propagation key.
It SHOULD return them in the same order as they appear in the carrier.
If the key doesn't exist, it SHOULD return an empty collection.

Required arguments:

• the carrier of propagation fields, such as an HTTP request.
• the key of the field.

The GetAll function is responsible for handling case sensitivity. If the getter is intended to work with an HTTP request object, the getter MUST be case insensitive.

Prior Art

#5973

[tongoss]

@MrAlias , can I pick this task?

[pellared]

I think that @jamesmoessis is willing to contribute as open-telemetry/opentelemetry-specification#4472 is merged. Am I correct?

[jamesmoessis]

@pellared yes I'm happy to finish this one off. I'm on leave this week but can do it in a week's time.

[MrAlias]

@jamesmoessis are you still able to work on this? We would like to include this in the next release.

[jamesmoessis]

@MrAlias apologies for the delay, I've just come back from leave post kubecon and Easter. So next week I was planning to finish this off. If that's not soon enough then feel free to grab it

[MrAlias]

Sounds good. Looking forward to the PR

[jamesmoessis]

PR is ready for review #5973