Add support for header in ErrorSource object (JSON:API 1.1 compliance)

[bansalHritik]

Description:
The current implementation of the ErrorSource class in src/JsonApiSerializer/JsonApi/Error.cs only includes support for Pointer and Parameter members.

According to the JSON:API v1.1 Specification [link], the source object of an error can now optionally include a header member:

source: an object containing references to the primary source of the error. It SHOULD include one of the following members or be omitted:
pointer: a JSON Pointer [RFC6901] to the value in the request document that caused the error [e.g. "/data" for a primary data object, or "/data/attributes/title" for a specific attribute]. This MUST point to a value in the request document that exists; if it doesn’t, the client SHOULD simply ignore the pointer.
parameter: a string indicating which URI query parameter caused the error.
header: a string indicating the name of a single request header which caused the error.