Add std::string::compare API to fixed_string [15351]

[richiware]

Description

Updating generated code from IDL files with last fastddsgen version breaks the compilation of several tests because
fixed_string doesn't implement std::string::compare. This PR implements this function.

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added.
• Any new/modified methods have been properly documented using Doxygen.
• Fast DDS test suite has been run locally.
• Changes are ABI compatible.
• Changes are API compatible.
• Documentation builds and tests pass locally.
• New feature has been added to the versions.md file (if applicable).
• New feature has been documented/Current behavior is correctly described in the documentation.

Reviewer Checklist

• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

[jsan-rt]

Little typo: std::string instead of std:.string

[richiware]

Done in a4d80e5

[MiguelCompany]

A test should be added to test/unittest/utils/FixedSizeStringTests.cpp

It would also be nice to have overloads for the compare method, similar to what we have for operator ==:

int compare(
        const char* s) const;

template<size_t N> int compare(
        const fixed_string<N>& str) const;

[richiware]

@richiprosima Please test this

[MiguelCompany]

LGTM

[richiware]

Failed tests not related.