feat: allow to add custom command dynamically

[SrinivasanTarget]

Change list

Appium 2.0 will allow custom plugins to add their own endpoint. This feature helps add custom commands dynamically.

Types of changes

What types of changes are you proposing/introducing to Java client?
Put an x in the boxes that apply

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Details

driver.addCommand(HttpMethod.GET, "/sessions", "getSessions");
driver.execute("getSessions");

[SrinivasanTarget]

@jlipps fyi

[mykola-mokhnach]

should we verify if the defined command does not override an existing one?

[SrinivasanTarget]

At server side of plugin, we allow to override the behaviour of an Appium command though the path of command remain same. Keep the client side in line with server, I would like to allow client plugins also to override the existing command in same path with a different behaviour.

[jlipps]

I think it would help to have an example of how an actual client plugin might be used by end users. This PR seems to help plugin builders, without necessarily providing a recommended format. For example, would it be something like this? (pseudo java):

CustomPlugin plugin = new CustomPlugin(driver);
driver.findElement(...); // do stuff with driver
plugin.myCustomCommand(...); // do stuff with plugin

Is there no way in Java to do some metaprogramming? Ideally we'd be able to use multiple plugins, something like this:

CustomPlugin1.installTo(driver);
CustomPlugin2.installTo(driver);
driver.findElement(...); // do stuff with driver
driver.customCommand1(...); // do stuff with plugin 1
driver.customCommand2(...); // do stuff with plugin 2

I don't know Java well enough to know whether this is possible.