fix(oob): allow encoding in content type header

[TimoGlastra]

Allows content type header to have encoding in the value. To not break implementations that only use application/json it just checks wether the header value starts with application/json

The oob RFC describes the content type header with encoding in it:

The sender MAY include a Content-Type header specifying application/json; charset=utf-8, and in the case where the agent included an Accept header for the application/json MIME type, the sender MUST include the header. If so, the sender MUST return the invitation in JSON format in the response body with a status_code of 200.

[codecov-commenter]

Codecov Report

Merging #1037 (61b580c) into main (34db14b) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main    #1037   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files         521      521           
  Lines       12130    12130           
  Branches     2003     2003           
=======================================
  Hits        10760    10760           
  Misses       1308     1308           
  Partials       62       62           

Impacted Files	Coverage Δ
packages/core/src/utils/parseInvitation.ts	75.00% <66.66%> (ø)

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