Add secure padding for save-image

[thetek42]

This PR adds support for "secure padding", just like esptool.py's --secure-pad-v2 option, as discussed in #713.

In contrast to main...SergioGasquez:espflash:feat/secure-padding, this also adds an additional padding section, just like esptool.py. Thus, using esptool.py and espflash save-image with --secure-pad-v2 will now result in exactly the same output.

Also, it makes the connection::reset crate public again, since it is required to call the Flasher::connect method when espflash is used as a library. It was previously made pub(crate) in be1ff81.

[SergioGasquez]

Hi! Here are my testing results.

# Generate the project
> esp-generate --chip=esp32s3 secure-pad (do not select any features)
# Generate the binaries
> esptools tool --chip esp32s3 elf2image --version 2 --secure-pad-v2 --output esptool.bin target/xtensa-esp32s3-none-elf/release/secure-pad
> espflash save-image --secure-pad-v2 --chip esp32s3 target/xtensa-esp32s3-none-elf/release/secure-pad espflash.bin
# Compare binaries
> esptools tool --chip esp32s3 image_info esptool.bin
...
7 segments

Segment 1: len 0x00350 load 0x3c000020 file_offs 0x00000018 [DROM]
Segment 2: len 0x00258 load 0x3fc89168 file_offs 0x00000370 [BYTE_ACCESSIBLE,MEM_INTERNAL,DRAM]
Segment 3: len 0x00400 load 0x40378000 file_offs 0x000005d0 [MEM_INTERNAL,IRAM]
Segment 4: len 0x00d68 load 0x40378400 file_offs 0x000009d8 [MEM_INTERNAL,IRAM]
Segment 5: len 0x0e8c8 load 0x00000000 file_offs 0x00001748 [PADDING]
Segment 6: len 0x013e8 load 0x42010020 file_offs 0x00010018 [IROM]
Segment 7: len 0x0ebc0 load 0x00000000 file_offs 0x00011408 [PADDING]
Checksum: 0e (valid)
Validation Hash: 894bf1f00093738fa4da353d4483d6d14969b0f657ccd6f4e6fff5f4d3342848 (valid)
Status Ok(ExitStatus(unix_wait_status(0)))
> esptools tool --chip esp32s3 image_info espflash.bin
...
6 segments

Segment 1: len 0x00350 load 0x3c000020 file_offs 0x00000018 [DROM]
Segment 2: len 0x00258 load 0x3fc89168 file_offs 0x00000370 [BYTE_ACCESSIBLE,MEM_INTERNAL,DRAM]
Segment 3: len 0x01168 load 0x40378000 file_offs 0x000005d0 [MEM_INTERNAL,IRAM]
Segment 4: len 0x0e8d0 load 0x00000000 file_offs 0x00001740 [PADDING]
Segment 5: len 0x013e8 load 0x42010020 file_offs 0x00010018 [IROM]
Segment 6: len 0x0ebc0 load 0x00000000 file_offs 0x00011408 [PADDING]
Checksum: 0e (valid)
Validation Hash: 865e0745cc17b5ea9c8e7e2e499e5f4cc6a774c490c286013aed72fa6ec11ebc (valid)
Status Ok(ExitStatus(unix_wait_status(0)))

cc @ivmarkov