Implement ExtendedTextMapGetter in instrumentations

[jamesmoessis]

Is your feature request related to a problem? Please describe.

Spec has recently changed to specify extraction of multiple values for a given key: open-telemetry/opentelemetry-specification#4295

The Java SDK has added ExtendedTextMapGetter to handle this: open-telemetry/opentelemetry-java#6852 (when stabilised this extension may become a default method on TextMapGetter which should be then implemented by instrumentations).

Describe the solution you'd like

For instrumentations that have carriers that support multiple values for a single key (most HTTP implementations) this should be a relatively straightforward change.

It does not necessarily need to be supported by all instrumentations.