Schema upgrade + x5c header adjustments + priority downgrade

[onematchfox]

• Schema upgrade fixes Upgrade plugin to make use of new schema format #11
• Header adjustments fix Make x5c claim optional #12 and Contents of x5c JWT header invalid #13

Also lowered priority of plugin to prevent it running:

• before other plugins such as request-transformer which may adjust the payload of the request to the upstream and thereby invalidate the payload hash calculated by this plugin
• prior to plugins such as rate-limiting which may invalidate the need to run this plugin entirely

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[onematchfox]

See this is now out of date with base branch and given the lack of response after being open for 11 months I'm going to close it out and continue with my own fork. Thanks for the initial work that went into this!