Create function to parse a hex string into a byte array (`TCMD_extract_hex_array_arg`)

[DeflateAwning]

• Needed for Issue #32.
• Needed to finish implementing the TCMDEXEC_flash_write_hex function, which currently throws an annoying build warning

uint8_t TCMD_extract_hex_array_arg(const char *args_str, uint8_t arg_index, uint8_t result_array[], uint16_t result_array_size);

Function should support ignoring spaces and underscores within the string, which are used as delimeters.

You may wish to factor out the hex parsing logic to one or more other functions, to separate it from the comma-searching nature of the TCMD_extract_ functions.

Requires a unit test.

Examples

• Invalid: 0x0000 -> do not allow "0x" prefix
• Valid: 0123FF -> simple example case, goes into length-3 byte array {0x01, 0x23, 0xFF} (which is the same as {1, 35, 255})
• Valid: 01 -> {0x01}
• Valid: 01_02_03 -> {1, 2, 3} (underscore can be a delimeter between bytes)
• Invalid: 010_203 -> underscore/space cannot be a delimeter in the middle of bytes (must be between even-numbered hex chars)
• Invalid: 1, 12345 -> must be an even length
• Invalid: 01_02_3 -> must be an even length

[robotoshi]

This is a duplicate of #77, but with more details

[parker-research]

Implemented in #111