Nov updates

[nsheff]

Codifying some recent decisions in the ADR and specification.

Specifically, this PR makes these changes:

• adds ADR for transient and passthru attributes
• adds new section to the spec for 'attribute qualifiers', which adds new information about passthru/transient attributes and also groups this with descriptions of inherent/collated qualifiers
• adds 'name_length_pairs' description as a RECOMMENDED attribute
• updates schema definitions to put attribute qualifiers under a 'ga4gh' key (Alignment: inherent property #84)
• fixes some links
• adds ADR for new 'minimal schema' where we changed the list of required/inherent attributes, and also updates the example schemas in the spec.

[nsheff]

Question:

Should we specify the qualifiers for the custom attributes?

sorted_sequences

• inherent: false
• collated: false
• passthru: false
• transient: false

sorted_name_length_pairs

• inherent: false
• collated: false
• passthru: false
• transient: true

name_length_pairs

• inherent: false
• collated: true
• passthru: false
• transient: false

Or are some of these allowed to vary by implementation?