Text-service output can use custom paths

[donaldgray]

What does this change?

Resolves #619 - allows control of how text-service paths, in line with https://github.com/dlcs/text-services/blob/main/instructions/alternative-paths.md

X-Forwarded-Host and X-Forwarded-Path are always set when making request. Former is set to Orchestrator URL, latter is set to default value, optionally overridden by TextServiceJob path template.

Additional Changes

Introduced new PathTemplate type, in favour of string for templates. Expresses expectations better and allowed PresentationPathReplacementHelpers and SupportedTemplateOptions to be stored in single place.

Added benefit is that call sites are now much cleaner:

// this
var path = PresentationPathReplacementHelpers.GeneratePresentationPathFromTemplate(template,
            customerId.ToString(), hierarchyPath, resourceId);

// becomes
var path = template.GeneratePath(customerId, hierarchyPath, resourceId);

Made 2 small tweaks in TypedPathTemplateOptions -

• Don't use static DefaultFormats as Defaults, create a copy instead. Currently changing the overridden values (set via appSettings) would override the baked-in presets.
• GetPathTemplatesForHost() uses Defaults rather than DefaultFormats to get all templates. Previously GetPathTemplatesForHost() would always used the DefaultFormats value, rather than Defaults. This is only used in PathRewriteParser

Configuration Changes

Optional TextServiceJob path template value. See path-customisation.md added with this PR for more details.