enable a pre-built bowtie2 index for rrna removal

[emmanuel-tan]

Summary (in progress)

Changes made

• bowtie2_rrna_index has been added to nextflow_schema.json with a brief description and help text

• Validation logic added in main.nf and workflows/rnaseq/main.nf to set a path to the prebuilt index if provided (initialise empty channel if not provided) and to set make_bowtie2_index to true

• No changes made to FASTQ_QC_TRIM_FILTER_SETSTRANDEDNESS or PREPARE_GENOME as the index building step in subworkflows/nf-core/fastq_remove_rrna/main.nf already checks if make_bowtie2_index is set to true before building

• Tests in development, need to source or build a bowtie index

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests! (In progress)
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/rnaseq branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).