agents: Fix agents package API

[tmc]

A few improvements to the agents package include:

• Executor -> *Executor
• Deprecate Initialize
• Rename CreationOption+CreationOptions to Option+Options
• Move agent-specific types/implementations to subpackages

[lujingxuansc]

I would appreciate it if this PR could be assigned to me.

[haochunchang]

Hi, I would like to ask what does "Rename CreationOption+CreationOptions to Option+Options” mean?
Does it means changing CreationOption type into Option?
For example:

// Changing this
type CreationOption func(*CreationOptions)
// into this
type Option func(*Options)

[tmc]

Hi, I would like to ask what does "Rename CreationOption+CreationOptions to Option+Options” mean? Does it means changing CreationOption type into Option? For example:

// Changing this
type CreationOption func(*CreationOptions)
// into this
type Option func(*Options)

Yes that's what I meant.