feat(gatsby,gatsby-adapter-netlify): support pathPrefix and trailingSlash options

[pieh]

Description

This adds support for pathPrefix and all trailingSlash options to gatsby-adapter-netlify.

On the adapter side:

• report support for pathPrefix and all trailingSlash options
• move built static files around before deploy so those static assets are routed to correctly (use Pretty URLs handling) - if expected route should be:
  • /prefix/page/ - then .html asset should be public/prefix/page/index.html
  • /prefix/page - then .html asset should be public/prefix/page.html
• cache and restore .cache and public locally (gatsby doesn't handle file moves and it expects artificats in same place, so we cache state of public before we do file moves, so when we restore it on future build assets are in places expected by gatsby)

There are some adjustments on Gatsby side as well:

• ensure paths in route manifest have the path prefix
• make ssr lambda able to resolve pages correctly when pathname with prefix is used

Tests

e2e-adapters in this PR start running 2 variants:

• (current) - no path prefix + default trailing slash (always)
• (new) - path prefix + never trailing slash

Various assertions and test setup were adjusted so same tests can be used for both variants (they will adjust their assertions based on variant settings).

Additionally this unskips the e2e tests for trailing slash behavior (and we test both always and never variants)

Related Issues

FRA-65
FRA-23