Add support for psr/http-message v2.0

[samuelszabo]

Problem

The library currently only supports psr/http-message ^1.0, which causes dependency conflicts when trying to install it in projects that use psr/http-message 2.0.

Error Message

Problem 1
    - lmc/matej-client[3.1.0, 4.0.0, ..., 4.1.0] require psr/http-message ^1.0 
      -> found psr/http-message[1.0, 1.0.1, 1.1] but the package is fixed to 2.0 
      (lock file version) by a partial update and that version does not match.

Solution

Update composer.json to allow both versions:

"psr/http-message": "^1.0 || ^2.0"

Why This Is Safe

The only breaking change in psr/http-message v2.0 is the addition of return type declarations to the PSR-7 interfaces. Since this library:

1. Only consumes the PSR-7 interfaces (uses them as type hints)
2. Doesn't implement or extend the PSR-7 interfaces directly
3. Already has return type declarations on its own methods

Both versions are compatible. The library will work correctly with either v1.x or v2.x.

References

• PSR-7 v2.0 breaking changes
• Similar issue in AWS SDK