feat: Allow add custom provider headers

[tienvx]

Add ability to add custom provider headers to v10. Somehow I didn't notice this feature is available in v9 and FFI at the same time.

[coveralls]

Pull Request Test Coverage Report for Build 10694982756

Details

• 25 of 25 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 97.342%

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Totals
Change from base Build 10632532438:	0.03%
Covered Lines:	2307
Relevant Lines:	2370

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💛 - Coveralls

[YOU54F]

Looks good,

I would have probably used a different example, rather than introducing a new one with some matcher todos. (Adding the form encoded example, may be better in a sep PR)

Update on docs would be nice - https://github.com/pact-foundation/pact-php/blob/master/docs/provider.md maybe pointing to this

https://docs.pact.io/provider/handling_auth#4-modify-the-request-to-use-real-credentials

Modify Requests Prior to Verification

and borrow some text from pact-js as to the rationale

https://github.com/pact-foundation/pact-js/blob/master/docs/provider.md#modify-requests-prior-to-verification-request-filters

I don't believe we have request filters as a concept in pact-php for modification of payloads prior to being sent to the provider. I believe JVM and JS do this by starting up a proxy.

[tienvx]

I added the doc and removed the form-urlencoded example. But I am not confident adding to any existing example