[Feature Request] Introduce more quantization parameter knobs

Description

When ran in CRF mode, SVT-AV1 assigns QP offsets per picture that are based on content / complexity etc having a range of QP offsets selected maybe quite large which would result in the ultimate bitrate being high even for CRF 63. Having additional knobs to reduce the bitrate would help in this use case.

Request One ffmpeg knob that, if implemented, would fit the list is b_qfactor but such could be generated for all frame types or planes for example:

--i_b-factor would change the maximum allowed distance q-index between the intra frames and b-frames. The encoder should still be generating content based qp offsets (we don’t want to ask the user to use fixed qp-offsets) but the CRF engine would take this parameter into consideration to reduce the distance between the qindex assigned to the key frame and the one assigned for the non reference frames. As a result, for example, when we select CRF 63, instead of having the non reference qp index be set to 255 and the key frame was decided by the encoder to be set to 155, this parameter would allow tell the encoder that we want the gap to be reduces by % percent and if set to 50 for example, the encoder would make the encoder use 50% of the range and set the key-frame to 205. Such may result in a hit to the quality, but would allow the user to achieve lower bitrates.

--luma_chroma-factor We would want to use this parameter to now increase the distance between luma and chroma. In a sense, we’d want to steal bits from chroma by applying a higher quantization step. Such would allow us to reduce the bitrate without potentially causing a major quality degradation.