Ena 6648 fasta reader

[IvaTutis]

A fasta reader that enables the user to read the sequences and stream/get as a string their slices.

Currently measured to process a 2GB single fasta entry with a large sequence in about 2.8 sec - so that's reading, parsing, indexing, complaining if the file is malformed etc etc.
Getting small slices of the sequence is almost instantaneous, while streaming the entire sequence will of course take longer, but around 3 sec is to be expected.

The user uses the reader through the interface in FastaFileService, the examples of usage are provided in FastaFileServiceIntegrationTest ("integration" because I am effectively testing the Fasta File Service together with the underlying SequentialFastaFileReader and the SequenceIndexBuilders and whatnot).

Requiring feedback on:

suggestions for a better interface in FastaFileService
structure
suggestions for optimizations
potential spotted bugs

The spec for the file I am reading is provided here.

Currently I am aware of the following potential bugs:

need check for trailing/leading Ns in case they span multiple lines
need to check if it's possible for there not to be a nextline character after the json in the header, and the sequence starts right after
probably should throw buffer sizes in some hardcoded constants file or appsettings

[Rajkumar-D]

Should we read from the end of the line to get the trailing N's?

[Rajkumar-D]

Had initial comments on this PR.