Improve escaping text in handlebars, with a focus on extensibility.

[Bpendragon]

These changes are based on those provided by @stoggi in #221 but with additional changes and movements based on suggestions from @crobibero.

Changes:

• Adds WebhookMediaContentType.cs an Enum that lists out the most common text encodings for webhooks (Plaintext, Json, and Xml)
• Sets most clients to use Json by default as that is what they are designed for.
• Defaults the Generic Client to Plain Text Encoding.
  • Adds logic to Generic Client to change the encoding based on the provided headers.
• Adds functions to BaseOption.cs to escape Json properly. Xml and Plaintext are left to use the existing escape function as they had been.
  • These can easily be changed to other or better functions in the future.
• Adds list of previously escaped text fields to BaseOption.cs
• Removes old escape function and calls from DataObjectHelpers.cs

This PR will:

• Supersede/Obsolete Fix discord json encoded strings #221
  • @stoggi is included as a Co-author on this PR for that reason.

This PR may:

• Solve/close Please provide a helper that supports changing to escape as \ n in the template #217

[crobibero]

Json is not assuimed in GenericFormClient, the content type should be application/x-www-form-urlencoded

[Bpendragon]

It may not be intended to be, but the body is currently treated solely as JSON per current master: https://github.com/jellyfin/jellyfin-plugin-webhook/blob/c507d/Jellyfin.Plugin.Webhook/Destinations/GenericForm/GenericFormClient.cs#L52-L58
Once in the processing itself the Generic Client does use FormURLContent (line 77 of same file, unchanged in PR)

[crobibero]

This should probably possible to set in the webui

[Bpendragon]

I do have it available to set in the Generic Client via a header, though at the moment it only supports json, xml, and generic/plaintext. For the individual pre-built clients though I think it makes sense to keep it hidden, or at least on a per-client basis. Discord webhooks for example only allow for JSON, so it makes no sense to expose an option that invariably breaks functionality if changed.