Extend CsvReader capabilities, move into new library

[mikee47]

This PR extends the capabilities CsvReader class, introduced in #2403.

Goals:

• Extend testing, verify efficient use of memory
• Add Parser capability to allow filtering and processing of (very) large files streamed via network, in files, etc.
• Add seeking support to allow indexing, bookmarking, etc.
• Add iterator support

The code has been moved into a separate library.

Changes:

Fix String move to avoid de-allocating buffers

Use longer of two buffers for result. Example:

String s1 = "Greater than SSO buffer length";
String s2;
s1 = ""; // Buffer remains allocated
s2 = std::move(s1); // The move we need to fix

At present this move will result in s1's buffer being de-allocated.
This can lead to performance degratation due to subsequent memory reallocations
where a String is being passed around as a general buffer.

This change uses the larger of the two buffers when deciding how to behave.
Checks added to HostTests to enforce predictable behaviour.

Add CStringArray::release method

Allows efficient conversion to a regular String object for manipulation.

Fix CStringArray operator+=

Must take reference, not copy - inefficient and fails when used with FlashString.

Move CsvReader into separate library

The CsvReader class has been moved out of Core/Data and into CsvReader
which has additional capabilities. Changes to existing code:

• Add CsvReader to your project's :cpp:envvar:COMPONENT_DEPENDS
• Change #include <Data/CsvReader> to #include <CSV/Reader.h>
• Change CsvReader class to :cpp:class:CSV::Reader

TODO:

• Check documentation
• Add sample to demonstrate iteration
• Is there a better way to deprecate current CsvReader.h other than an error? Yes: static_assert.