Move internal method contextFromString out of JaegerSpanContext

[isaachier]

Requirement - what kind of business use case are you trying to solve?

Currently, the internal contextFromString method resides in the JaegerSpanContext class. It produces a JaegerSpanContext instance from a string that conforms with the TextMapCodec. Arguably, this is not an ideal location for it to reside.

Problem - what in Jaeger blocks you from solving the requirement?

In the future, we would like to support other codecs, such as W3C trace identifiers (jaegertracing/jaeger#855). This method implies the only valid codec is the TextMapCodec seeing as it is the canonical method for using strings to produce JaegerSpanContext and implicitly relies on the TextMapCodec format.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Move the contextFromString method into the TextMapCodec class instead.