Support autodetection for macromolecules file formats

**Background**
Currently convert api method allows to detect file format automatically if parameter input_format: chemical/x-unknown passed.
However for sequence and fasta formats it is impossible to identify which type of sequence need to be converted (peptide, rna or dna).

**Solution**
Introduce new parameter monomer_type: and format autodetection for sequence ans fasta formats.

Start at the convert API's existing input_format autodetection path and inspect how sequence and fasta formats are handled. Define the monomer_type parameter with RNA, DNA, and PEPTIDE values, then verify that autodetection distinguishes these sequence types and that conversion succeeds for each format.

Written by the indexing model from the issue text.