Revisit HttpHeadersAssert API with more header-focused assertions

[simonbasle]

In 6.2.x, HttpHeadersAssert inherited AbstractMapAssert assertions. Since 7.0, HttpHeaders is no longer a Map and we should take that as an opportunity to introduce more header-focused assertions (potentially inspired by the Map ones).

In #33913, we already introduced the following:

• containsOnlyHeaders
• hasExactlyValues
• hasExactlyValuesInAnyOrder

Some assertions that were ported over in #33913 might not be that relevant and should be re-evaluated as part of this effort:

• size assertions between/lesser/greater
• ...

[snicoll]

I can see that hasEntrySatisfying is gone. I think it would be helpful to be able to run assertions on the value(s) of a header based on its name.

[simonbasle]

An hasEntrySatisfying-like assertion would indeed be useful 👍

the current work is available at main...simonbasle:spring-framework:gh-34168