Convenience functions for `cxx::string`

## Brief feature description

This is an issue to collect convenience functions for the `cxx::string` which would be nice to have but are not urgently needed.

## Detailed information

- add c'tor that constructs the `iox::string` with count copies of a character
```
iox::string<10> str(TruncateToCapacity, count, 'M'); // count as runtime parameter
iox::string<10> str(count, 'M'); // count as compile time constant
```
- remove characters from the `iox::string`
```
iox::string<10> str("someString");
str.erase(5, 2); // str == someSing
str.erase(7); // str == someSin
```

Locate the implementation and tests for iox::string first. Review the requested count-and-character construction forms and erase examples, then verify completion against both API behaviors, including runtime and compile-time counts and removal ranges.

Written by the indexing model from the issue text.