[Flight] Allow passing DEV only startTime as an option

[sebmarkbage]

When you use the createFromFetch API we assume that the start time of the request is the same time as when you call createFromFetch but in principle you could use it with a Promise that starts earlier and just happens to resolve to a Response.

When you use createFromReadableStream that is almost definitely the case. E.g. you might have started it way earlier and you don't call createFromReadableStream until you get the headers back (the fetch promise resolves).

This adds an option to pass in the start time for debug purposes if you started the request before starting to parse it.

[react-sizebot]

Comparing: 58bdc0b...f57283f

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	605.41 kB	605.41 kB	=	107.21 kB	107.22 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	664.38 kB	664.38 kB	=	117.09 kB	117.09 kB
facebook-www/ReactDOM-prod.classic.js	=	688.25 kB	688.25 kB	=	121.13 kB	121.13 kB
facebook-www/ReactDOM-prod.modern.js	=	678.67 kB	678.67 kB	=	119.48 kB	119.49 kB
oss-stable/react/cjs/react.react-server.development.js	=	31.64 kB	30.93 kB	=	7.53 kB	7.39 kB
oss-stable-semver/react/cjs/react.react-server.development.js	=	31.61 kB	30.90 kB	=	7.50 kB	7.36 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
facebook-react-native/react/cjs/React-dev.js	+0.56%	52.61 kB	52.91 kB	+0.57%	11.61 kB	11.67 kB
oss-experimental/react/cjs/react.development.js	=	51.34 kB	50.63 kB	=	11.55 kB	11.40 kB
oss-stable/react/cjs/react.development.js	=	49.19 kB	48.48 kB	=	11.16 kB	11.02 kB
oss-stable-semver/react/cjs/react.development.js	=	49.17 kB	48.46 kB	=	11.14 kB	10.99 kB
facebook-www/React-dev.classic.js	=	56.01 kB	55.17 kB	=	12.32 kB	12.19 kB
facebook-www/React-dev.modern.js	=	56.00 kB	55.17 kB	=	12.32 kB	12.18 kB
oss-experimental/react/cjs/react.react-server.development.js	=	39.21 kB	38.50 kB	=	9.18 kB	9.04 kB
oss-stable/react/cjs/react.react-server.development.js	=	31.64 kB	30.93 kB	=	7.53 kB	7.39 kB
oss-stable-semver/react/cjs/react.react-server.development.js	=	31.61 kB	30.90 kB	=	7.50 kB	7.36 kB

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