Added onCapture function parameter, and customisable capture button

[Shamsudeen12]

Description

This PR adds a function parameter "onCapture" to allow the user to perform an action immediately after taking a photo. It also allows the user customise the capture button What your Pull Request change ?

Checklist

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• [ x] 📕 I read the Contributing page.
• [x ] 🤝 I match the actual coding style.
• [x ] ✅ I ran flutter analyze without any issues.

Breaking Change

• [ ] 🛠 My feature contain breaking change.

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[Shamsudeen12]

Alright, will do @g-apparence

[Shamsudeen12]

@g-apparence I have made the changes you requested

[Juanchote]

when are you guys going to merge this into master (for 2.0.0)?