[WIP] Set up a Rust native version of Hardsubx context and rewrite hardsubx.c

[shashwat1002]

• constructor function (new) made that takes options as argument [this will replace init_hardsubx]
• default function for hardsubx context
• default function for cc_subtitle

[shashwat1002]

@PunitLodha please check if the new struct looks okay

[PunitLodha]

@prateekmedia Since you made the change to rsmpeg. Is it a drop-in replacement? Are there any changes needed to this PR?

[prateekmedia]

@PunitLodha you can check what the CI says for build_ocr_hardsubx linux, if it runs fine then no change else I think there might be some imports that need to be done.

Like ffmpeg_sys_next::* would not be equivalent to rsmpeg::*, some dependencies are needed to be include manually. Check my implementation on mod.rs and utility.rs

[cfsmp3]

@prateekmedia @IshanGrover2004 @PunitLodha thoughts on this PR?

[PunitLodha]

Code looks fine, but it needs testing before it can be merged. @prateekmedia @cfsmp3