Added TLS & credentials configuration for NATS_consumer input plugin

[mmelnyk]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

Basic changes to add missing TLS & credentials options for NATS. Mutual TLS is supported too.

[danielnelson]

I've got thinking about this a bit more and realize that we still need the secure option, otherwise it isn't possible to connect with a default tls.Config. Normally it would be controlled by the URL in a HTTP input (http vs https), or by using a enable_tls option, but since we already have secure I added it back in ffe9494.

[mmelnyk]

There are some inconsistencies in options for other plugins - e.g. nats output (btw, readme for this plugin needs updating) does not have "secure" option, but usually used in a pair with nats_consumer.

[danielnelson]

I added the same option to the output too now, technically this is a breaking change as you now must set the secure option to use TLS, it's not enough to just use non default settings.

When it comes to plugin inconsistency, yeah it's quite difficult to keep up, especially considering the number of plugins these days. Definitely relying on the community to open PRs.