Allowed payload to be replaced in EndpointRequestDefinition

[Steveb-p]

Question	Answer
JIRA issue	N/A
Type	feature/bug/improvement
Target eZ Platform version	v4.5
BC breaks	no
Doc needed	no

This PR allows payload to be replaced when creating EndpointRequestDefinitions in a data provider.

First of all, it introduces the withPayload method, which returns a clone of a request definition with changed payload.

Second, payload name is now always set by the factory. This prevents an issue where - since name is used as data provider dataset key - changing the request payload caused data provider to contain multiple entries under the same key, resulting in PHPUnit error.

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly (master for features, the oldest supported for bugs).
• Asked for a review (ping @ibexa/engineering).