add optional getAll method to propagation api

[brettmc]

Implement open-telemetry/opentelemetry-specification#4295 which was added to spec 1.40.0:

Adds optional GetAll method to Getter in Propagation API, allowing for the retrieval of multiple values for the same key.

The spec further suggests that the implementation should be non-breaking if possible. Java adds an ExtendedPropagationGetterInterface which can optionally be implemented by propagators.

edit:
It looks like getAll is designed to handle repeated headers, and specifically calls out that baggage could appear multiple times in an HTTP request. However, RFC-2730 s3.3.2 says that:

A recipient MAY combine multiple header fields with the same field
name into one "field-name: field-value" pair, without changing the
semantics of the message, by appending each subsequent field value to
the combined field value in order, separated by a comma.

nyholm/psr7 certainly merges the headers per RFC: https://github.com/Nyholm/psr7/blob/1.8.2/tests/RequestTest.php#L186
Based on that, I think that there is nothing to do here since our implementation already meets the objectives of getAll.